1. Field of the Invention
The invention relates to a method for producing oil by injecting into an oil-bearing formation a slug of CO.sub.2 followed by a fluid drive. An improvement is to inject prior to the CO.sub.2 slug, a slug containing a water soluble salt capable of reacting with calcium ions from the formation to produce an insoluble salt which precipitates on mineral surfaces, thereby inhibiting mineral dissolution and preserving more of the CO.sub.2 for oil displacement.
2. Background of the Invention
In the recovery of oil from oil-bearing formations, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the formation. Thus, a variety of supplemental recovery techniques such as thermal recovery methods, waterflooding and miscible flooding have been employed in order to increase the recovery of oil from subterranean formations.
One technique of enhanced recovery is to inject into the formation a slug of CO.sub.2 to mobilize oil, and subsequently inject a driving fluid to displace the oil-rich and CO.sub.2 phases toward a production well. One of the major drawbacks or disadvantages in the use of CO.sub.2 in such a miscible flood process is the incidental consumption or loss of CO.sub.2 in the formation. When the connate brine of an oil-bearing formation is contacted by CO.sub.2 in the course of CO.sub.2 flooding operations, carbonic acid is formed which, by ionization reduces the prevailing pH, i.e., EQU H.sub.2 O+CO.sub.2 .revreaction.H.sub.2 CO.sub.3 .revreaction.H.sup.+ +HCO.sub.3.sup.-
This in turn reduces the existing carbonate ion concentration, which causes carbonate minerals, e.g., calcite, previously in equilibrium with the aqueous phase, to dissolve until a new equilibrium is established, i.e., EQU H.sup.+ +CO.sub.3.sup.= .revreaction.HCO.sub.3.sup.- EQU CaCO.sub.3 .fwdarw.Ca.sup.++ +CO.sub.3.sup.=
In addition to causing non-essential and uneconomical consumption of CO.sub.2, such dissolution may produce undesirable changes in formation permeability by creating channels, especially during the early stages of the CO.sub.2 flood, or by plugging existing permeability during the late stages. See G. D. Ross et al, "The Dissolution Effects of CO.sub.2 -Brine Systems on the Permeability of U.K. and North Sea Calcareous Sandstones", Society of Petroleum Engineers, U.S. Dept. of Energy 10685, Tulsa, April, 1982.
The present invention provides a method for more effectively utilizing CO.sub.2 in combination with a driving fluid in an oil recovery process and thereby enhancing oil recovery. The invention comprises injecting aqueous solutions containing a water-soluble salt comprising sodium oxalate or sodium fluoride preceding or simultaneously with the CO.sub.2 so as to protect the formation mineral matrix from attack by carbonic acid generated in-situ, thereby preserving more of the CO.sub.2 for oil displacement and preventing undesirable changes in formation permeability.